Association between Condylar Bone Changes and Eichner Index in Patients with Temporomandibular Dysfunction: A Cone Beam Computed Tomography Study

Statement of the Problem: Eichner index is a dental index, which is based on the occlusal contacts between naturally existing teeth in premolar and molar regions. One controversial topic is the association between occlusal status and temporomandibular joint dysfunction (TMD) and its associated degenerative bony changes. Purpose: Through the use of cone-beam computer tomography (CBCT), the current study sought to ascertain the relationship between the Eichner index and condylar bone alterations in TMD patients. Materials and Method: In this retrospective study, the CBCT images of bilateral temporomandibular joints (TMJs) of 107 patients with TMD were evaluated. The patients’ dentition was classified into three groups of A (71%), B (18.7%), and C (10.3%), according to the Eichner index. Radiographic indicators of condylar bone alterations, including as flattening, erosion, osteophytes, marginal sclerosis, subchondral sclerosis, and joint mice, were either present or absent and registered as 1 or 0, respectively. Chi-square test was used to evaluate the link between the condylar bony changes and the Eichner groups. Results: According to the Eichner index, the most prevalent group was group “A”. The most prevalent radiographic finding was “flattening of the condyles” (58%). Condylar bony changes were found to be statistically related to age (p= 0.00). However, no significant relationship was found between sex and condylar bony changes (p= 0.80). There was a significant relationship between the Eichner index and condylar bony changes (p= 0.05). Conclusion: Patients with greater loss of tooth supporting zones have more condylar bony changes.


Introduction
The temporomandibular joint (TMJ), one of the body's most intricate articulations, has a wide range of anatomical and physiological characteristics [1]. Different factors can affect the articular surface of the condyle. Because of continuous stimulation, the TMJ constantly remodels [2]. Adaptive responses of bone are flattening of the bony surface or marginal sclerosis in response to loading forces that exceed the normal tolerances of joint tissues. When the ability of the joint to excessive loading forces through remodeling is surpassed, other degenerative changes include subchondral sclerosis, osteophyte, joint mice, and Ely's cyst take place. These changes in the condyle can predispose temporomandibular dysfunction (TMD) [2][3][4].
TMD describes a group of clinical complaints that affect the stomatognathic system, mainly the muscles of mastication [5]. TMD has a multifactorial etiology. It has been found that several risk factors are associated with the symptoms and signs of TMD [6]. A number of links have been reported between dental status and TMD. The loss of teeth, especially posterior teeth, can end up in an increase in the occlusal load on the remaining teeth and appears to disturb the function of TMJs in later stages [7]. A clinical examination alone cannot adequately assess the osseous and soft tissues of the TMJs [8]. Therefore, radiographic evaluation of the osseous structures is required to detect possible destructive changes in the TMJs in patients with TMD. Various radiographic modalities have been introduced to evaluate condylar morphology [9]. It is likely that CBCT, a newly developed imaging technique in dentistry, will be the most effective method of assessing the osseous morphology of TMJ joints.
Developed by Karl Eichner, the Eichner index can be applied for epidemiological studies and it is one of the most widely used dental indices. This index is effective in establishing intermaxillary contacts and extending functional dental invalidity [10]. Using this index, the posterior teeth are divided into four support zones based on whether occlusal contact exists between the premolars and molars.
The relationship between Eichner index and bony changes in the condylar region has only been studied in a few studies [6,29]. In these studies, however, bony changes in the condyle were evaluated using twodimensional images. Thus, the purpose of this study was to determine whether condylar bone changes are associated with Eichner index in TMD patients using CBCT.

Materials and Method
In accordance with IR.SUM.DENTAL.REC. 1399.09-08, the present study was approved by the Institutional Research Committee. In this study, 107 patients with clinical signs and symptoms of TMD who needed further CBCT investigation were recruited from the archives of a private clinic and an oral and maxillofacial radiology department. There was a wide age range of 16 to 80 years for the patients (39.57±3.31 years). Exclusion criteria were patients with an established history of temporomandibular surgery, acute trauma, congenital abnormalities, musculoskeletal or neurological diseases, and any systemic diseases potentially affecting joint morphology.

Tooth loss recordings
Based on the occluding pairs in the posterior teeth (two premolars and two molars), the dentition of each patient was divided into four main occlusal supporting zones.
All of the four supporting zones are in contact in class A; one supporting zone is missing in class B, or all of the four supporting zones are absent, but the anterior region remains intact; and class C has no occlusal contact between the remaining teeth ( Figure 1). This study considered both fully and partially erupted permanent teeth as "present teeth". Moreover, the supernumerary teeth, third molars, pontics of bridge prostheses, and implant-supported superstructures were not counted as the present teeth. The raw data were reconstructed primarily for the TMJ. By scrolling the axial images, the system identify-  For each patient, the right and left TMJ areas were evaluated for the presence or absence of one or more of these radiographic changes, and were rated accordingly as 1 or 0.We calculated the left and right TMJ bony changes independently by adding up the scores related to any radiographic finding. For example, when a patient had flattening, erosion, and sclerosis in the condylar bones, the total number of changes was considered "3". The right and the left scores were then added. Consequently, the association between the condylar bony changes and the Eichner index was examined.

Measurement precision
The images were evaluated by two dentomaxillofacial radiologists. Each observer evaluated the images independently after a minimum of two weeks. In addition to checking the consistency between the first and the second sets of records produced by each specialist, we also examined the reliability of the inter-examiners for each of the criteria applying κ statistic. Adequate intraexaminer agreement index (0.831 to 1 and 0.833 to 1 for two specialists), as well as strong inter-examiner agreement (κ coefficient: 0.801 to 1) was detected.

Discussion
A common cause of orofacial pain that is not related to dental or infectious conditions is TMD [12]. Several factors contribute to the symptoms and signs of TMD.
Since posterior teeth are necessary to maintain uniform occlusal force distribution on TMJ, losing them can have a greater impact than occlusion type. Additionally, due to incisors' inclined planes, entire mandible move backward when posterior teeth are lost. As a result of this movement, the condyles are moved above and behind their normal position in relation to the articular eminence. When a unilateral cause is responsible, only one condyle is affected but when the bite has a closed, both are affected [27][28].
Only two studies have investigated the relationship between Eichner index and condylar bony changes [6,29].
A significant correlation was found between Eichner index and condylar total bony changes in the present study. Findings of this study conflict with those of Takayama et al. [6] and Jalalian et al. [29]. According to Takayama et al. [6], despite the positive correlation between the Eichner index and the symptoms of TMD, A negative correlation was found between the Eichner index and the prevalence of bone changes in the condyle. The Eichner index and condylar bony changes were also not significantly correlated in a study by Jalalian et al. [29]. The variation in the findings in these studies can be explained by the limitations of the conventional radiographic modality used (panoramic radiography). Approximately 75% of condylar osteoarthritis changes detected by CT can be missed with panoramic  the earliest alteration and bone change that occurs on articular surfaces in progressive diseases. It is thought that flattening is caused by the involvement of the masseter and temporal muscles, which overload the TMJ [12]. Unlike this study, Jalalian et al. [29] and Takayama et al. [6] found that Ely's cyst is most frequently seen on condylar radiographs.  [36]. Alzaharani et al. [35], Takayama et al. [6], and Jalalian et al. [29] reported a similar result. This is contrary to the findings of Gharge NR et al. [27], who found a higher prevalence of condylar bone alteration in women.
Various radiographic modalities such as plain film radiography [27,29], conventional tomography [37], computed tomography (CT) [38][39], CBCT [4,40], and magnetic resonance imaging (MRI) [41][42] have been used in previous studies to evaluate condylar morphology. In conventional radiography, such as panoramic radiography, the most lateral portion of condyle is represented as a 2-dimensional image. Using these radiographs to assess condylar morphology is therefore less reliable. The 1-3mm thickness of conventional slices limits the accuracy of assessing changes in condylar morphology. MRI is a method used to assess the soft tissue parts of the TMJ. The use of CT is widely used in medicine as a diagnostic tool, but its availability, cost, and radiation exposure limit its use in dentistry. In comparison with spiral CT, CBCT has a sub-millimeter spatial resolution that is as high as or higher than spiral CT [43]. Therefore, CBCT was chosen as the preferred imaging modality.
This study aimed to determine whether radiographic bone changes in the condyles are associated with Eichner; however, some factors such as the time between tooth extraction and image time and oral habits were not evaluated in relation to the severity of TMD symptoms and condylar bony changes. To either confirm or refute the results of the present study, further investigation should be conducted with these factors taken into account.

Conclusion
The results of this study indicate that condylar bony changes are highly associated with the Eichner index. In addition, CBCT findings were associated with variations in finding condylar bony changes in this study. It is therefore important to consider CBCT as a modality for appropriate diagnosis in clinical practice as well as for patients' therapeutic choices.

Conflict of Interests
The authors declare that they have no conflict of interest.